This application claims the priority of German Patent Document 199 34 855.3, filed Jul. 24, 1999, and PCT/EP00/05587, filed Jun. 17, 2000, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a composite shaft/disc workpiece assembled by friction welding and to a method for producing the same.
In vehicle and machine building, composite workpieces composed of a shaft and of an essentially rotationally symmetrical disc are employed in numerous applications. Examples of such discs are gear and clutch parts or else disc-shaped blanks which still have to be machined mechanically.
European Patent Document EP 372 663 A1 discloses a method, with the aid of which a shaft and a disc can be connected in a connection region by means of friction welding. For this purpose, the shaft is provided in the connection region with a conical or stepped outer surface, while the disc is provided with a conical or stepped passage hole corresponding to the connection region on the shaft. This configuration of the connection region has the effect of centering the shaft in relation to the disc during the friction-welding operation. When the shaft is being welded to the disc, either two conical individual surfaces of approximately negative shape meet one another, so that the friction-welded component has a coherent conical connection surface, or a plurality of radially adjacent planar annular surfaces offset in the axial direction meet one another, so that a connection surface composed of a plurality of planar steps is obtained. In both cases, a large-area connection region which is approximately free of cavities is obtained.
With a view to saving weight in engines, transmissions, etc., it is advantageous to use composite shaft/disc workpieces, the shafts of which are designed as hollow shafts. The method described in EP 372 663 A1, in which the steps or the cone in the connection region serve(s) for centring the shaft in relation to the disc and in which a large-area connection region between the shaft and the disc is obtained, cannot be employed, however, particularly in the case of thin-walled hollow shafts, since the wall of the hollow shaft is softened as a result of the large-area heating during friction welding, to an extent such that, when the disc is pressed on, deformation of the shaft wall is initiated. In the method of EP 372 663 A1, therefore, the hollow shaft cannot oppose the disc in the connection region with sufficient pressure resistance for welding, and the disc can be slipped over the connection region, instead of making a fixed rigid connection with the hollow shaft.
Furthermore, particularly in the case of shafts rotating at high speed, for example in transmissions, it is necessary to achieve a further saving of weight and a low mass moment of inertia in relation to the axis. There is therefore a great need for composite shaft/disc workpieces which have a reduced weight, in particular, even in regions distant from the axis, that is to say even in the region of the disc, and at the same time a high design strength of the shaft/disc connection must be ensured.
The present invention provides a composite shaft/disc workpiece which has a reduced weight, as compared with the prior art, and has a low mass moment of inertia in relation to the axis of rotation. Further, a method for producing such a composite workpiece is provided.
Accordingly, the shaft is provided, in a connection region in which the disc is to be welded to the shaft, with a plurality of rotationally symmetrical steps, the diameter of which becomes increasingly large in a direction of the axis. On the other hand, the disc is provided with a passage hole in the region of the axis of rotation. The passage hole is configured in such a way that it has a plurality of rotationally symmetrical webs which project from the disc towards the axis of rotation. The inside diameter of the webs is selected such that each web forms a passage hole, the inside diameter of which is smaller by a welding overlap than the step which is located in the connection region of the shaft and which is opposite this web in the assembled position. The disc is assembled, together with the shaft, by friction welding. At the same time, the webs are connected to the steps, and, in the region of the webs, annular weld seams are obtained which, depending on the size of the welding overlap, have a more or less conical shape. Annular cavities are obtained at the same time between successive webs.
Considerable weight savings can be achieved by means of these cavities, as compared with a conventional filled connection region. In particular, the mass moment of inertia of the composite workpiece composed of the shaft and the disc can thereby be greatly reduced. A high rigidity of the connection is achieved by means of the annular weld seams which lie parallel to one another. Good welding quality is ensured when the welding overlap of the webs in relation to the steps is about 1 mm to 3 mm. The weld seams formed in this case cover a region of 5 mm to 15 mm in the axial direction of the composite workpiece and are slightly tilted conically in relation to the axial direction of the composite workpiece.
In order to achieve the greatest possible saving of weight of the composite workpiece composed of the shaft and disc, it is advantageous to design the shaft as a hollow shaft. In order to avoid a radial bulging of the hollow shaft in the direction of the axis during friction welding, particularly when a hollow shaft of small wall thickness is used, the interior of the hollow shaft is provided in the connection region with a supporting element, by means of which deformations of the hollow shaft are prevented.
A particularly large saving of weight and reduction in the mass moment of inertia is achieved when the disc is composed of a conical sheet-metal dish and of a supporting frame, the supporting frame having a plurality of annular supporting webs, to which the sheet-metal dish is fastened. The supporting webs give the sheet-metal dish rigidity, while the weight of the disc is reduced by means of the cavities located between the supporting webs. The sheet-metal dish and the supporting structure may in this case consist of different materials. Thus, the sheet-metal dish may be manufactured from a carbon steel which is capable of tolerating the highest possible compressive and frictional forces, while the supporting structure is produced from a suitable lightweight material. The webs of the supporting frame are expediently connected by friction welding to the sheet-metal dish wall located opposite them.
The cavities formed between the webs and the outer wall of the shaft in the assembled position may expediently be used for carrying liquid and gaseous media. In particular, they serve for the radial (circulatory) distribution of lubricating oil or pressure oil which is carried via oil ducts inside the shaft and is guided onto the outer surface of the shaft at defined locations in the surroundings of the disc. The annular cavity in this case makes it possible to have a substantial simplification in the geometry and production of the oil ducts. Arranging the bores in pairs opposite one another reduces unbalance.
The method according to the invention makes it possible to connect shafts to discs made of different materials. In particular, it allows a reliable assembly of a disc consisting of any desired friction-weldable material together with a hardened steel shaft. The disc and shaft can thus be ready-machined separately and hardened, as required, before they are assembled to form a composite workpiece.